Known scrubbers comprise a housing having at least one inlet hole and an outlet hole and comprising filtering material. A scrubber may for example be used in any equipment where for example NO-free air is needed, e.g. for calibration, as in an equipment for measuring the level of nitric oxide in air, especially exhalation air, or any other gas mixture. In such a case a test person inhales air through the scrubber so that NO will be filtered from the air. The air flow may for example be about 1–10 liter/second.
Preferably a return valve is positioned between the inlet/outlet of the equipment, through which inlet/outlet the test person inhales/exhales, and the outlet of the scrubber so that the exhaled air does not pass the scrubber but reaches any sensing portion of the equipment.
When taking a zero reference for the sensing portion of the equipment or making a function control, a small flow of component-free air, for example NO-free air, is used and taken between the outlet of the scrubber and the return valve. One problem with this is that the possibility exists that the zero reference will be mixed with a leakage back flow through the return valve. This would compromise the accuracy of the zero measurement.
This is of course true for any filtering of a component from air. Therefore, it is an object of the present invention to provide a scrubber, which safely makes sure that the zero reference flow actually comes from the scrubber and is identical to the component-free air inhaled by the test person.